The present invention relates generally to a head-up display apparatus for an automotive vehicle for displaying information, such as vehicle speed, engine speed, fuel supply, engine coolant temperature, or so forth. More specifically, the invention relates to a display luminosity control circuit for controlling the brightness of the projected display of information on a front windshield in driver's field of view.
Such apparatus is used, for example, to provide the driver of a vehicle with a display of information which he can consult without having to divert his line-of-sight from the scene ahead of the vehicle. The head-up display includes generally a display-producing means for producing the display information instead of or in addition to a speedometer, a tachometer, a fuel gauge, a temperature gauge, or the like arranged on an instrument panel.
Japanese Patent Publication No. 57-182540 exemplifies a conventional head-up display apparatus as described above.
It is necessary for the display to compensate for the brightness of the image projected onto the windshield in accordance with variations in the foreground light level. Therefore, the brightness of the display is controlled in response to the foreground light level conditions.
In one such control system, in view of the inevitably required ON and OFF headlight switching operations due to sudden variations of foreground light level when the vehicle goes into a tunnel and comes out of it, an ON and an OFF signal of a headlamp switch is used as a detecting signal for detecting variations in the foreground light level. Thus, this system can vary the brightness of the display without imposing a burden on the driver.
In another system, a photo sensor is used, as a detecting means for detecting variations in the foreground light level, mounted above an instrument panel. The system controls the brightness of the display in accordance with a signal output by the sensor. According to the second system, the brightness of the display is varied continually in accordance with variations in the ambient light level without imposing a burden on the driver.
In the above described first and second systems, the detecting methods for detecting variations in the ambient light level differ. These systems, however, control the brightness level of the display in response to the detecting signal for indicating variations in the ambient light level.
However, the above control systems vary the brightness of the display immediately in response to variations in the foreground light level conditions. The following problems are encountered.
When an automobile goes into a tunnel in the daytime, a conventional control system reduces the brightness of the display in response to darkness in the tunnel. The driver's eyes, however, do not adapt to the reduced brightness of the image immediately, so that the displayed information cannot be visibly identified until the driver's eyes adjust.
When the automobile comes out of the tunnel, the system increases the brightness of the display in response to the foreground light level. The driver's eyes, however, cannot adapt to the increased brightness of the display immediately. The display is, therefore, momentarily too bright and dazzling.